Superplastic forming is one of the most important technologies to emerge in recent years. This technology uses a phenomenon called superplasticity which occurs in several metals under well-defined conditions of microstructure, temperature, and strain rate. The most important characteristic of superplastically formable materials is their exceptional stability in uniaxial tensile deformation. This enables extremely large delongations, usually greater than 200 percent without fracture; whereas, for conventional materials the equivalent values are usually less than 50 percent. Since the potential for large elongations in several structural metals, primarily titanium alloys, was first demonstrated, the superplastic forming of such alloys has been systematically developed into a technology for manufacturing parts on a production basis.
Forming methods employing the principle of superplastic metal forming use fluid pressure to cause sheet material deformation into a die complementary to the part to be formed. Several methods have been described for superplastically forming parts using various techniques with fluid pressure whereby the pressure is generated by gas under pressure on one side of the sheet. The opposite side of the sheet is maintained either under atmospheric pressure, or sometimes a vacuum is generated on this side. As the pressure increases and/or the vacuum increases, the sheet is deformed into substantial engagement with the surface of the die. However, many reject parts result due to partial destruction of the part caused by cracks being formed as the strain rate exceeds the critical range permissible, thereby resulting in the loss of superplasticity in the sheet and permitting rupturing caused by concentrating all subsequent sheet deformation in one or more local areas. Also, many rejects occur due to incomplete forming of the component relative to the die cavity.